Binocular Vision &

Eye Muscle Surgery Qtrly°

5.

Cover Test findings:

The cover test should be measured

while the patient views an accommodative

target at 6 meters and at 40 cm. The

average deviation when the patient fixates

at 6M is 29A and at 40 cm it is 9 pd (31).

The distribution of angular measure-

ments is leptokurvic resulting in few XTs

being larger than 50 pd or smaller than 10 pd

White (61) has noted that the greatest

deviation occurs when viewing at 200 ft.

Intermittent values occur while viewing at

60 ft.

Most X(l)s, who have never been

treated, think that they know or “feel”

when their eye is deviating or straight.

However, casual observation demonstrates

that their perception of alignment or

deviation is inaccurate.

Ogle & Dyer (38) used fixation disparity

methods to provide accurate information

about oculomotor coordination while

fusion was being maintained. According to

their studies, the oculomotor balance

obtained during dissociation testing, i.e.,

cover test, does not indicate true binocular

imbalances. They reported that during

fusion the oculomotor imbalances found in

DEX(T) were often normal and/or the

associated phoria was often an esophoria.

In no case was the associated phoria close

to the magnitude of a dissociated

phoria/tropia.

Surgery, according to Ogle & Dyer (38)

did not alter the fixation disparity curve

but did affect the measurement obtained

by dissociated means. They concluded that:

‘The tropia found by clinical disassociation

tests are a manifestation of an innervation

or oculomotor imbalance not present when

fusion is maintained”.

6.

Concomitancy:

Although most X(T)s demonstrate rela-

tive comitancy, Moore et al (62) reported

about 24% of their patients had lateral

gaze incomitances, i.e., limitation in

abduction. According to Moore et al, those

patients with lateral gaze incomitance were

eight times more likely to have a surgical

overcorrection.

Kushner (41), who cites Morton, states

that only 5% have lateral incomitance.

Furthermore, Repka & Arnoldi (63) sug-

gest that many lateral incomitancies are

due to errors in measurements from im-

proper prism positioning.

A vertical deviation in the tropic eye

has been found in 45-55% of all DEX(T)s

during distance fixation (64,65). This

vertical deviation is often similar to Bell’s

phenomenon in that it does not occur

upon initial deviation, but only after the

Major Review: Intermittent Exotropia;

Basic and Divergence Excess Type

J.

Cooper, MS. OD and N. Medow, MD

fully manifested deviation has occurred.

Jampolsky (66) feels that the superior

rectus (SR) muscle is a stronger elevating

muscle than the inferior oblique (10)

muscle. Thus, during manifest XT, the

adducted SR might cause an elevation or

hyperdeviation of the deviating eye. The

incidence of overaction of the inferior

oblique muscles (IOOA) in DEX(T),

according to Davies (67), is 30%. Similar

findings have been reported by Wilson &

Parks (68) who noted that 32% of all

X(T)s have IOOA which they first noted

around 5 years of age. Davies (67) re-

ported that the vertical deviation was more

often found in distance testing and absent

in near testing. Of the patients having a

vertical deviation as identified by Davies,

62.8% had a primary vertical deviation

without IOOA 32.5% had a primary ver-

tical deviation with IOOA, while 4.6% had

non-dissociated vertical deviation.

7.

Temporal characteristic:

Most exotropic deviations are inter-

mittent and alternators (of fixation). Alter-

nation takes approximately 80 msec (69).

This is approximately the same time for a

large saccade.

Schlossman & Boruchoff (32) reported

that 85% of all XTs are intermittent, 9%

are constant alternators, and 6% are

amblyopic. They were the first to use the

term intermittent exotropia to include the

majority of DEX(T)s and basic X(T)s.

The first author (1) has suggested that

many of the previously classified constant,

alternating or minimally amblyopic XTs

were most likely intermittent if examined

carefully. One must note the position of

the eyes before disassociating with a cover

test, since an X(T) may be inadvertently

broken down by occluding the eye during

visual acuity measurements. They will be

incorrectly classified as a constant XT.

Amblyopia associated with X(Y) is

usually secondary to anisometropia (not

strahismu.s) and minimal in amount (65).

The deviation is usually more latent at

near than at distance. Burian & Smith

(70) reported that the angle of deviation

increases by 25% when changing fixation

from 20 to 100 feet. Also, approximately

10% dramatically change their temporal

characteristic, e.g., become constant. The

difference in frequency noted between dis-

tance and near fixation has been ascribed

to: high AC/A, proximal convergence, slow

vergence (vergence aftereffects), larger

retinal disparity, enhanced binocular visual

acuity and/or greater angle subtended by

fusional stimuli (1). As previously men-

tioned, the AC/A probably has minimal

Summer of 1993

Volume 8 (No.3): 185-216

effect on either the magnitude or fre-

quency of the deviation.

The deviation can be triggered by a

myriad of situations and stimuli, such as

inattention, daydreaming, distance viewing,

fatigue, ifiness or bright sunlight. Deviating

in sunlight, resulting in closure of an eye is

so common in XT, that any child with a

history of monocular closure of an eye

must be assumed to have an XT until

otherwise proven.

This photophobia from a photic stim-

ulus leading to eye closure has been

assumed to be due to a dazzling of the

retina so that fusion is somehow lost (17).

Wirtshafter & von Noorden (71) have

shown that light adversely affects fusional

convergence in X(T). Additionally, monoc-

ular closure has been induced by arti-

ficially increasing illumination. Eustace &

Wesson (17) postulated that sunlight

causes an X(1) since, statistically, the

closer to the equator one lives the greater

the incidence of X(T). Romano (18) has

suggested that elimination of sunlight by

hats with brims might eliminate or reduce

the deviation. Wang & Chiyssanthau (72)

investigated the relationship between

anomalous retinal correspondence (ARC)!

normal retinal correspondence (NRC) and

monocular eye closure. Monocular closure

in X(T) occurred in 90% of the patients

with NRC and only 35% of the patients

with ARC. They concluded that this is

evidence that eye closure is performed to

avoid diplopia and confusion. However,

this is conjecture since DEX(T) patients

with NRC do not complain of diplopia or

confusion. (Also, high correlations do not

prove cause and effect.)

Recently Wiggins & von Noorden (73)

evaluated the report of diplopia as the

reason for monocular eye closure in

DEX(T). They also video-recorded eye

closure responses to bright light in an

attempt to determine if the deviation

occurred prior to closure of the eye. They

reported that closure occurs before

deviation and that no subject perceived

diplopia. Contrary to the report of Wang

& Chryssanthau (72), they found no re-

lationship between retinal correspondence

and eye closure. Wiggins & von Noorden

also did not find any effect on fusional

convergence with increased illumination.

Wiggins & von Noorden (73) concludeth

“No satisfactory explanation can be offered

at this time to explain the high prevalence

of monocular eye closure in intermittent

exotropia”.

An alternate explanation might be that

bright light decreases contrast and

therefore fusional detail resulting in an

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